Hewitt Industries of El Seguendo, California manufactures a vehicle speed control unit comprising a normally closed solenoid controlled on-off valve in a bypass line. When the vehicle speed exceeds a predetermined maximum speed the solenoid is energized to open the valve and divert some of the fuel through the bypass line back to the fuel tank, but only if the fuel delivery pressure is above a predetermined minimum pressure. The control circuit for the solenoid includes a first switch controlled by a vehicle speed sensor and a second switch controlled by pressure in the bypass line. The vehicle speed sensor senses a vehicle overspeed condition and in response closes the first switch. If the second switch is closed at the time, the solenoid is energized, causing it to open the on-off valve in the bypass line. This allows some of the fuel in the delivery line to flow back to the fuel tank, with the remainder of the fuel being delivered to the engine. Under some conditions of vehicle operation this bypass of fuel reduces the amount of fuel that is delivered to the engine below what is necessary to provide sufficient fuel at the engine for cooling the fuel injectors. The pressure responsive switch senses this condition. When there would be an insufficient delivery of fuel to the engine, during an overspeed condition of the vehicle, the pressure responsive switch opens the circuit, removing energy from the solenoid and allowing the off-on valve to close. This results in an immediate buildup in pressure and a functioning of the pressure switch to again activate the solenoid so as to again open the off-on valve. The problem with this type of system is that the constant turning of the off-on valve accelerates wear and greatly reduces the life of the valve.
A principal object of the present invention is to provide a bypass type speed control system which includes a solenoid controlled off-on valve in a bypass line but which is turned on by an overspeed condition of the vehicle and remains on until the vehicle speed is adequately reduced, while maintaining adequate fuel delivery to the fuel injection.